1. Field of the invention.
The invention relates to a magnetic record/playback system for a helical-scan magnetic record/playback apparatus (referred to as a helical-scan VTR) and, more particularly, to a magnetic record/playback system suitable for use with a magnetic record/playback apparatus (referred to as a VTR) in which a television picture signal is PCM-modulated and recorded, and is then reproduced through inverse processes.
2. Prior Art
VTRs of this type have been widely used for home-use picture recording apparatus. Recent remarkable developments in peripheral circuitry have solved technical problems, for example, large time-axis jitter, and have improved the characteristics of VTRs. Home-use VTRs recently available on the market, accordingly, have excellent characteristics in comparison with four-head VTRs for broadcasting purposes, and feature compact size, light weight, low cost and the like.
Development of the helical-scan VTR has placed emphasis on improvements in its characteristics. However, it is required that the characteristics and operability of the VTR be further improved so that it can be used in additional applications. In order to use a VTR of this type in the broadcasting field, an editing function is essential and, in addition, the function of reproducing a still picture or slow-motion picture is also required.
Various types of record/playback systems for helical-scan VTRs have been proposed which modify the way in which the tape is wound or wrapped around a head drum or the number of magnetic heads used. One of these modifications is the SMPTE (Society of Motion Picture and Television Engineers) system. The SMPTE system and the technical problem involved will briefly be described.
In the record playback system for the type-C VTR of SMPTE (SMPTE Journal: vol. 87, Mar., 1978; C98. 18, C98. 19, pp. 163-165), a magnetic tape is scanned in a manner such that signals in one field are recorded in response to one rotation of a magnetic head, thus avoiding so-called in-picture switching. Here, the in-picture switching means switches a reproduction or playback head during a horizontal blanking interval other than the vertical blanking interval. For this reason, the magnetic tape is scanned by winding the tape substantially entirely around a head drum or cylinder. Therefore, friction between the drum and the tape is large and this frequently impedes travel of the tape. Further, such a structure makes it difficult to load the tape. Particularly, when the magnetic tape used is a cassette tape, the tape must be pulled out of the cassette after loading in a VTR and then wound around the head drum. Accordingly, a very complicated structure is required for performing this operation. In an alpha type or an omega type wrap format VTR in which the magnetic tape is wound around the head drum by approximately 360.degree. or more, it is difficult to realize a simple and small-sized record/playback structure for a cassette tape.
In a type-B record/playback system of SMPTE (SMPTE Journal: vol. 87, February 1978; C98. 15, C98. 16, pp. 89-90), the magnetic tape is wound around the head drum over an angle of approximately 180.degree., with first and second heads separately disposed with the same heights in the rotational-axis direction of the head drum. With such a construction, a television picture signal (a video signal) of one field is recorded on five tracks of the tape during 2.5 rotations of the head drum. Accordingly, the first and second heads scan the same track when the tape stops travelling so that only 1/5 of one field video signal is played back for one head rotation. Therefore, an external memory with a large memory capacity and means for moving the tape in a reciprocal manner must be provided for playing back slow-motion pictures or still pictures. This results in the need for various kinds of drive and control circuits, leading to complex circuit construction.